The present invention relates to a device for the electrical detection or identification of ions for the mass spectroscopic determination of the mass values and of the mass intensities of the ions. The device is provided with a channel multiplier plate having a number of electron multipliers of minimum dimension located behind the discharge or exit gap of the magnet of the mass spectroscopic device for capturing (collecting) ions focused in the focal plane of the mass spectroscopic device. The device has, further, means for capturing (collecting) the electrons emanating from the channel multiplier plate and emitted or knocked off by ions of different mass in the electron multipliers of the channel multiplier plate, and for transmitting the signals generated by the collected electrons to a recording device series-connected to the device.
Arrangements for the detection of ions in the spectroscopic determination of the mass values and/or of the mass intensity of the ions were used, for example, in spark mass spectrometry or in secondary mass spectrometry. Frequently an attempt is made to cover the mass spectrum as completely as possible.
It is already known in the art how to identify ion current signals incident during a mass spectroscopic measurement (test) by means of a photo plate located in the focal plane of the mass spectroscopy device. All ion current signals are recorded simultaneously which is advantageous especially when ion currents which greatly fluctuate timewise are involved. However, this advantage is diminished by the fact that the emulsion of the photo plate has only a relatively small threshold. Therefore, in case all measurable mass values of the spectrum are to be evaluated, several exposures of the same photo plate with different exposure times are required. For evaluation of certain mass lines, one can use only those exposures where the mass lines are in the evaluatable range of the nonlinear sensitometric curve of the photo plate. Hence the evaluation of the spectrum is cumbersome and time-consuming. Another disadvantage is that the measuring accuracy of the photo plate is impaired by the variation of the sensitivity of the photo plate from plate to plate, and frequently within one and the same plate and by the even greater variation from emulsion to emulsion. Furthermore, the absolute sensitivity of the photo plate depends on many factors such as ion mass, ion energy, ion type, charge condition of the ion and the form of the ion. Therefore, photo plates are generally used only for measuring relative ion frequencies of occurrence.
It is also known in the art how to record mass spectra by means of devices where the ion current signals are measured electrically. If only one collection point for an ion current signal is provided at the discharge gap of the mass spectroscopic device, the mass spectra are recorded by slowly varying the magnetic separating field or the accelerating potential in the mass spectroscopic device. Consequently, the ion current signals reach the collection point successively in time. In contrast to the identification of the ion current signals by means of a photo plate, with the electrical identification of the ion current signals, the mass size is directly proportional to the intensity of the ion current signal and hence can be measured with great accuracy for all mass values of the spectrum. The disadvantage is that with this known device for the electrical detection of the ion current signals, the ion current signals arrive one after the other at the collection point. As a result, the measuring accuracy of this known device is limited due to the statistical fluctuations of the signals. As a consequence, with ion currents of less than approximately 10.sup.-.sup.14 amperes, only relatively inaccurate results can be achieved. To be sure, in such cases the measurements are repeated several times and the measured values are integrated; however, the relative accuracy then depends heavily on the measuring time.
There are also known devices with at least two collection points for the ion current signals which are used when the ratio of frequencies of the isotopes is to be measured. The spacing between the collection points corresponds to the mass ratio of the isotopes to be measured. This results in increased measurement accuracy; but this known device records only the ion current signals of the isotopes to be measured, and not the entire mass spectrum.
There also is known a device for the electrical detection or identification of ions where behind the discharge or exit gap of the mass spectroscopic device, there is located a channel multiplier plate known as "Spiraltron array" (cf. "International Journal of Mass Spectrometry and Ion Physics", 11, pp. 409 - 415, 1973). Behind the channel multiplier plate whose length is approximately 3 cm, an anode with a resistance plate of constant thickness is located at a distance of 0.25 cm. The impact points for the electrons exiting behind the channel multiplier plate and impacting the anode are obtained from the ratio of the parts of the resistance plate located on both sides of the impact point towards the two ends of the anode. The disadvantage is that the determination of the impact points is restricted by the fluctuations in the linearity of the resistance coating and by the exit divergence of the electron beams up to the anode. The local resolution, which is stated to be several tenths of a millimeter, is relatively small. Another disadvantage is that the impact points of simultaneously arriving electron beams cannot be determined.
It is, therefore, an object of the present invention to provide an arrangement for the detection or identification of ions in the mass spectroscopic determination of the mass values and/or the mass intensities of the ions which makes it possible to record the spectrum of the ion current signals with a great local resolution, and at the same time to identify all measured ion current signals with great accuracy.
Another object of the present invention is to provide an arrangement of the foregoing character which is simple in design and construction, and which may be economically fabricated.
A still further object of the present invention is to provide an arrangement, as described, which has a substantially long operating life.